Method and arrangement for calibrating an arangement for driving image-reproducing means subject to inertia

ABSTRACT

In a method and arrangement for correcting an arrangement for driving imagereproducing means subject to inertia, and particularly liquid crystal displays, wherein a stored correcting variable is added to infed video signals to compensate for the effects of inertia, which correcting variable depends on changes in the video signals from frame to frame, and wherein the corrected video signals are conveyed to the image-reproducing means, provision is made for a test pattern that contains signal jumps that occur from frame to frame to be generated, for the signal jumps to vary in respect of their sign, their size and their position in the amplitude range of the video signals, for the test video signals to be shown on the imagereproducing means at least in a part that is covered by at least one opto-electrical sensor, and for correcting parameters to be derived from the signals generated by the at least one optoelectrical sensor while taking account of the totality of the signals generated by the at least one opto-electrical sensor.

The invention relates to a method and arrangement for calibrating anarrangement for driving image-reproducing means subject to inertia, andparticularly liquid crystal displays, wherein a stored correctingvariable is added to infed video signals to compensate for the effectsof inertia, which correcting variable depends on changes in the infedvideo signals from frame to frame, and wherein the corrected videosignals are conveyed to the image-reproducing means.

Liquid crystals displays (LCDs) are well known for having anunsatisfactory time response. When an infed video signal jumps betweentwo successive frames, this does not result in a corresponding jump inthe luminance emitted by the LCD. Instead, the liquid crystal displayshows a noticeable inertia, which means that the luminance emittedapproaches the preset value only gradually. The transition may be drawnout over a plurality of refresh cycles. In image sequences depictingmotion, this characteristic results in motion-related errors, which takethe form in particular of edges being reproduced with a blur. Themotion-related error depends on the amplitude of the video signal thatis current at the time and on the previous video signals. Also, theluminance response of the liquid crystal display depends on the specifictechnology that is used in the particular case.

Because of the blurred edges of moving objects in image sequences, thiseffect will also be referred to below as motion blur. In a method thathas become known from, for example, U.S. Pat. No. 6,304,254 B1, theframe that is previous at the time is stored. The values of theindividual pixels in the current frame and the previous frame areentered in a table and from the table is read a correcting variable thatcauses a jump in the video signal to be overdriven.

In what follows, the correcting variables required for all the signaljumps will also be referred to, as a whole, as a compensating scheme.The compensating scheme depends on various factors including, amongstothers, the temperature of the liquid crystal display. The fact oftemperature being taken into account when applying the compensatingscheme is already known from U.S. Pat. No. 6,304,254 B1, where the valueat the time of the ambient or liquid temperature is applied to an extrainput of a table. Although this gives temperature-dependent control ofthe compensating scheme, it does not provide complete calibration.

Calibration is achieved by the method according to the invention bygenerating a test pattern that contains signal jumps that occur fromframe to frame, by varying the signal jumps in respect of their sign,their size and their position in the amplitude range of the videosignals, by showing the test video signals on the image-reproducingmeans at least in a part that is covered by at least one opto-electricalsensor, and by deriving correcting parameters from the signals generatedby the at least one opto-electrical sensor while taking account of thetotality of the signals generated by the at least one opto-electricalsensor.

The method according to the invention has the advantage that the actualoutput variable from the liquid crystal display, namely its luminance,is used to calibrate the compensation scheme.

To save storage space, the method according to the invention may be soarranged that of all the possible signal jumps, only selected datumvalues are used to construct the test pattern.

The method according to the invention may be further developed bycausing the calibration to be performed each time the image-reproducingmeans is switched on. To allow for changes that take place over thecourse of operation, provision may also be made for the calibration tobe repeated at preset intervals of time.

Alternatively, the method according to the invention may also be soarranged that the temperature of the image-reproducing means is measuredat at least one point on the means and is stored at the time of acalibration, and that a further calibration is performed if there arechanges in the measured temperature that exceed a preset thresholdvalue. One temperature sensor may be provided in this case, or aplurality of temperature sensors may be arranged to allow for the curvefollowed by temperature in the vertical direction and, if required, inthe horizontal direction too. The curve found for temperature can thenbe used as well to form the compensation scheme.

The method of calibration according to the invention can be appliedtogether with various methods of compensation. A particularlyadvantageous combination comprises, to allow the correcting variable tobe formed, providing a model of the image-reproducing means thatcontains the correcting parameters, which model has a state variable asits output variable, the video signals as a first input variable and thestate variable from a previous frame as a second input variable, and,also to allow the correcting variable to be derived, using a table thathas the infed video signals and the state variable from the previousframe as its input variables and the corrected video signals as itsoutput variable. The model may also be incorporated in the table in thiscase.

The state variable in this case is a numerical representation of avariable derived from the curve followed by luminance over time that iscaused by signal jumps. This variable may for example be the luminanceat the end of a refresh cycle or the mean of luminance over a refreshcycle.

An application employing a different method of compensation comprises,to allow the correcting variable to be derived, providing a tablecontaining the correcting parameters, which table has the infed videosignal and the video signal for the previous frame as its inputvariables and the correcting variable as its output signal.

To enable the compensation set by the calibration to be monitored duringthe ongoing operation of the image-reproducing means, provision may alsobe made in the method according to the invention for, during the showingof video signals of any desired kind on the image-reproducing means, thesignals generated by the opto-electrical sensor to be compared with thevideo signals of any desired kind, and for a calibration to be performedif there are wide deviations in respect of time response.

The invention also covers an arrangement for performing the methodaccording to the invention which comprises arranging the at least oneopto-electrical sensor at the edge of the image-reproducing means.

In an advantageous embodiment of this arrangement, provision is made forthe at least one opto-electrical sensor to be arranged outside the imagearea of the image-reproducing means and for an optical means to beprovided to guide the light from the image area to the opto-electricalsensor. This largely rules out the possibility of part of the imagebeing hidden by the opto-electrical sensor. The means for guiding thelight may be glass fibers or semi-transparent mirrors.

Another embodiment comprises making the at least one opto-electricalsensor pivotable. In this way, the sensor can be totally removed fromthe field of vision if required.

Finally, provision may be made in the arrangement according to theinvention for a plurality of opto-electrical sensors to be arranged atdifferent points at the edge of the image area. Allowance can be made inthis way for local differences in the behavior of the liquid crystaldisplay. This is for example the case when the liquid crystal display isexposed to different mechanical stresses.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 is a diagrammatic representation of an arrangement for carryingout the method according to the invention,

FIG. 2 shows a known arrangement for compensating for the effects ofinertia,

FIG. 3 shows a particularly advantageous arrangement for correcting theeffects of inertia, and

FIGS. 4 to 6 show various arrangements of the opto-electrical sensors.

FIG. 1 shows the liquid crystal display 1 in diagrammatic section. Thevideo signals Vi to be displayed are conveyed to the liquid crystaldisplay 1 via an input 2, a means 3 for compensating for the effects ofinertia and a changeover switch 4. The means 3 contains a compensationscheme, also referred to as an overdrive scheme, which will later beelucidated in detail in connection with FIGS. 2 and 3. A systemcontroller 5 controls the running of the method according to theinvention and generates data for a test pattern generator 6 and thecompensation scheme, which data is fed to the means 3. At an input 7,the system controller receives synchronizing information belonging tothe video signals Vi fed in at 2.

In the region of the bottom edge of the liquid crystal display 1 issituated an opto-electrical sensor 8 that converts light generated inthis region of the liquid crystal display into electrical signals andfeeds the latter to the system controller 5. Situated at the rear of theliquid crystal display 1 are a plurality of temperature sensors 9 to 12whose outputs are likewise connected to the system controller.

For calibration, the system controller 5 switches the changeover switch4 to its upper position. It can remain in this position during entirerefresh cycles, so that the test pattern is visible over the entireimage area, or for only part of the refresh cycles, so that the testpattern only appears in portions that are covered by the opto-electricalsensor.

The test pattern generator 6 is controlled in such a way that itgenerates signal jumps whose sign, amplitude and position within theamplitude range change and which take place from frame to frame. Thecurve followed by luminance is measured for each of these signal jumpsby means of the sensor 8. This preferably takes place over a pluralityof frames because the reaction of the luminance of the liquid crystaldisplay also extends over a plurality of frames. Once the successiveresponses of the luminance to the signal jumps generated have beenmeasured and stored, the system controller 5 calculates from them acompensation scheme that is recorded in the means 3.

The compensating arrangement shown in FIG. 2 has an input 2 for theinfed video signals Vi, which signals make their way via an adder 22 toan output 23 and are fed from there to the liquid crystal display ascorrected video signals Vo. The video signals are in the form of digitalsignals, with a value being assigned to each pixel. These values arestored in a store 25 for one frame at a time and, simultaneously withthe values A for the previous frame read out from the frame store 25,are fed as input variables to a look-up table (overdrive LUT). For eachpair A, B, the latter contains a value of the correcting variable C,although for reasons of storage space only datum values can be storedand the rest of the values are obtained by interpolation.

The correcting variable C taken from the look-up table 24 is selected tobe such that motion blur is compensated for as satisfactorily aspossible, and it is conveyed to the adder 22. As can be seen from FIG.2, in obtaining the correcting variable it is only the previous framethat is allowed for apart from the current frame.

It is true that motion blur can be improved to a first approximationwith the arrangement shown in FIG. 2, but the arrangement does havevarious disadvantages. In this way, overdriving is not possible at theboundaries of the amplitude range of the amplifiers of the liquidcrystal display for example. If however there is no overdriving for thisreason then, because only one frame is stored, it is also impossible fora subsequent correction to be made after a jump of this kind. Thesedisadvantages are avoided by the arrangement shown in FIG. 3.

In the compensating arrangement shown in FIG. 3, the corrected valuesB+C of the video signals Vo are fed to a model 26 of the liquid crystaldisplay. The model represents the luminance response of the liquidcrystal display to the video signal that is being fed to it at the timeand is therefore designated a “response model”. Its output variable S isstored in the frame store 25. The variable S′ for the previous frame,which is read out of the frame store 25, is used as an input variable tothe model 26 in addition to B+C. What is thus obtained is a recursivestructure, thus enabling a plurality of preceding frames to be takeninto account in deriving the correcting variable C. Like the variable Awhich was described above in connection with FIG. 2, S′ is fed to thelook-up table together with the B values.

In the embodiment shown in FIG. 4, an opto-electrical sensor 32 is soarranged that it does not obstruct vision of the image area 31 of theimage-reproducing means 1. A thin light guide 33 is provided whichpasses the light generated in a sub-area of the image area 31 to theopto-electrical sensor 32.

FIG. 5 shows an embodiment having a pivotable opto-electrical sensor 34which can be pivoted in front of the bottom part of the image area, atthe time of a manually initiated correcting operation for example.

In the embodiment shown in FIG. 6, there are four opto-electricalsensors 35, 36, 37, 38 provided which receive light from the edge of theimage area 31 at the top and bottom. In this way, allowance can be madein the correction for behavior by the liquid crystal display thatdiffers at different edges.

1. A method of calibrating an arrangement for driving image-reproducingmeans subject to inertia, and particularly liquid crystal displays,wherein a stored correcting variable is added to infed video signals tocompensate for the effects of inertia, which correcting variable dependson changes in the video signals from frame to frame, and wherein thecorrected video signals are conveyed to the image-reproducing means,characterized in that a test pattern is generated that contains signaljumps that occur from frame to frame, the signal jumps vary in respectof their sign, their size and their position in the amplitude range ofthe video signals, the test video signals are shown on theimage-reproducing means at least in a part that is covered by at leastone opto-electrical sensor, and correcting parameters are derived fromthe signals generated by the at least one opto-electrical sensor whiletaking account of the totality of the signals generated by the at leastone opto-electrical sensor.
 2. A method as claimed in claim 1,characterized in that, of all the possible signal jumps, only selectedones are used as datum values for forming the test pattern.
 3. A methodas claimed in claim 1, characterized in that the calibration takes placeeach time the image-reproducing means is switched on.
 4. A method asclaimed in claim 1, characterized in that the calibration is repeated atpreset intervals of time.
 5. A method as claimed in claim 3,characterized in that the temperature of the image-reproducing means ismeasured at at least one point thereon and is stored at the time of acalibration, and in that a further calibration is performed if there arechanges in the measured temperature that exceed a preset thresholdvalue.
 6. A method as claimed in claim 1, characterized in that, toallow the correcting variable to be formed, there is provided a model ofthe image-reproducing means that contains the correcting parameters,which model has a state variable as an output variable, the videosignals as a first input variable and the state variable from a previousframe as a second input variable and, also to allow the correctingvariable to be derived, a table is used that has the infed video signalsand the state variable from the previous frame as input variables andthe corrected video signals as an output variable.
 7. A method asclaimed in claim 1, characterized in that, to allow the correctingvariable to be formed, there is provided a table that contains thecorrecting parameters, which table has the infed video signal and thevideo signal for the previous frame as input variables and thecorrecting variable as an output signal.
 8. A method as claimed in claim1, characterized in that, during the showing of video signals of anydesired kind on the image-reproducing means, the signals generated bythe opto-electrical sensor are compared with the video signals of anydesired kind, and in that a calibration is performed if there are widedeviations in respect of time response.
 9. An arrangement forcalibrating an arrangement for driving image-reproducing means subjectto inertia, and particularly liquid crystal displays, wherein a storedcorrecting variable is added to infed video signals to compensate forthe effects of inertia, which correcting variable depends on changes inthe video signals from frame to frame, wherein the corrected videosignals are conveyed to the image-reproducing means, and wherein atleast one opto-electrical sensor detects at least a part of a testpattern that is shown on at least a part of the image area of theimage-reproducing means, characterized in that the at least oneopto-electrical sensor is arranged at the edge of the image-reproducingmeans.
 10. An arrangement as claimed in claim 9, characterized in thatthe at least one opto-electrical sensor is arranged outside the imagearea of the image-reproducing means and an optical means is provided toguide the light from the image area to the opto-electrical sensor. 11.An arrangement as claimed in claim 9, characterized in that the at leastone opto-electrical sensor is pivotable.
 12. An arrangement as claimedin claim 9, characterized in that a plurality of opto-electrical sensorsare arranged at different points at the edge of the image area.